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WASHINGTON — He has played the role of Hillary Rodham Clinton, Al Gore and Barack Obama. He has spent hours studying intricate details of their policy positions, perfecting the cadences of their voices and refining the lines of attack against their Republican opponents.

For more than a decade, Senator Rob Portman, Republican of Ohio, has deployed an unusual expertise: impersonating Democrats to help candidates in his own party prepare for debates. He has frustrated George W. Bush, agitated Dick Cheney and pushed John McCain beyond the brink of irritation.

“I hate him still,” Mr. McCain said with an admiring air of sarcasm, laughing in a brief interview as he recalled their testy practice sessions four years ago. “Because he’s such a jerk, he gets under your skin, says things that are impossible to respond to.”

But this year, Mitt Romney may have another part in mind for Mr. Portman.

The search for Mr. Romney’s running mate is entering its final phase, and Mr. Portman, who has ascended to senator from staff member during nearly three decades in Washington, is believed to be high on the list of prospects. He has campaigned repeatedly with Mr. Romney, and he is flying to New Hampshire this week — where the Romney family is vacationing — to appear at a Republican fund-raiser.

Mr. Portman, who was elected to the Senate in 2010 after representing the Cincinnati area in the House for 12 years, is relatively unknown to most Republicans across the country. But here in Washington, where he worked in both Bush administrations and has won praise as an effective legislator, he is among a select breed of politicians who have a keen and deep understanding of the inner workings of government.

When he arrived in the Senate, he startled some Republican colleagues when he pulled out his own charts and graphs that analyzed the nation’s rising deficits and its debt of $15 trillion. His earnestness initially rubbed a few people the wrong way, particularly because of his tenure as budget director for President George W. Bush.

At a time when fiscal concerns weigh heavily on the electorate, and the tug of war between the Republican establishment and the Tea Party still simmers, many conservatives still blame Mr. Bush and his team for allowing increases in government spending and budget deficits that they consider excessive. In interviews, several friends argued that since Mr. Portman served for only a year as director of the Office of Management and Budget, he did not bear responsibility for the administration’s full record.

If Mr. Romney is looking for a stirring orator to help electrify the ticket, Mr. Portman may not be his first choice. But for a governing partner with strong relationships in Congress, Republicans believe Mr. Portman is far more attractive.

The vetting of possible vice-presidential candidates has been under way for months. It is a deeply secretive process overseen at the Romney campaign headquarters by one of the candidate’s most trusted confidantes, Beth Myers. Last month, Mr. Romney tried to discourage speculation by declaring, “There are only two people in this country who know who are being vetted and who are not.”

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Senator Rob Portman of Ohio after a meeting on deficit reduction. He is considered a candidate for Mitt Romney’s running mate.Credit
Brendan Hoffman for The New York Times

Republicans close to Mr. Romney believe Mr. Portman is on the list, along with a handful of others including former Gov. Tim Pawlenty of Minnesota, Representative Paul D. Ryan of Wisconsin and Gov. Bobby Jindal of Louisiana. The roster could be larger, associates of Mr. Romney caution, but many Republicans argue that Mr. Portman is ideally suited for campaigning in Ohio and other critical Midwestern states.

“He would probably be the best-prepared person of anyone since George Herbert Walker Bush became Ronald Reagan’s vice president,” said Joe Hagin, who also worked in both Bush administrations and has known Mr. Portman since childhood. “He has an immense understanding of the budget, the tax code and trade and foreign policy.”

Mr. Portman, 56, has close ties to the Bush family. Barbara Bush recorded radio advertisements that helped him win an early Ohio Congressional race. This week in Maine, Mr. Portman is scheduled to have lunch with the elder George Bush, whom he served in the White House as associate counsel and head of legislative affairs.

Friends of both men say they are not certain if Mr. Bush has shared his admiration for Mr. Portman with Mr. Romney — any such talk would be private, presumptuous or both, one associate said. But the former president praised Mr. Portman when asked about him last week.

“Rob keeps telling me I’m the guy who got him into all of this,” Mr. Bush said in a statement. “All I know is I am very proud of him and his record. If you ask me how and when we met, I don’t remember when I met anyone anymore. I just know that I’ve known him a very long time.”

The Romney campaign and its team of lawyers have probably combed through every facet of Mr. Portman’s career, which began in Washington after he graduated in 1984 from the University of Michigan Law School. He worked as a lobbyist at Patton Boggs, an influential firm in the capital, and worked at two Ohio law firms between stints in the two Bush administrations.

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No Republican has ever been elected president without winning Mr. Portman’s home state, but it is unclear how much he would lift the ticket. He won his Senate race two years ago by 18 percentage points, but a Quinnipiac University poll in May found that 59 percent of Ohio voters did not know enough about him to form an opinion.

The chemistry between Mr. Romney and Mr. Portman was on display on a recent bus tour across Ohio. After they bounded onto the stage together in Newark, Ohio, last month, Mr. Romney beamed as Mr. Portman assailed the economic record of the Obama administration. The two men have an easy rapport, aides said, which was strengthened when Mr. Portman endorsed Mr. Romney during the primary.

Mr. Portman can deliver a speech in Spanish. He learned the language while living on a ranch near the Rio Grande in Texas during a college break, working alongside illegal immigrants. The experience intrigued him enough that he used it to write his anthropology thesis at Dartmouth.

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Mr. Portman at the Ohio River Way Paddlefest in Hamilton, Ohio, last month. He is relatively unknown across the country.Credit
Kirk Irwin for The New York Times

He recorded radio ads in Spanish during his Senate race and promoted his jobs plan to Hispanic voters. With a light accent and a trill of his R’s, he introduced himself by saying, “Soy Rob Portman, candidato para el Senado de los Estados Unidos.”

Mr. Portman, who has returned to Cincinnati on weekends during much of his time in Washington, has been married to his wife, Jane, for 25 years. She once worked for a Democrat, Tom Daschle, during his early days representing South Dakota in the House. She agreed to become a Republican when her husband agreed to become a Methodist before their marriage in 1986. They have three children.

Mr. Portman’s friends say he has started to become exasperated at being labeled by comedians and commentators as boring. Stephen Colbert, on a recent episode of his Comedy Central program, said a Romney-Portman ticket would be “the bland leading the bland.”

But many Republicans say that is precisely Mr. Portman’s attraction, as he would provide an antidote to Sarah Palin, the party’s last vice-presidential candidate. He has delivered a series of self-deprecating jokes to combat being described as dull. But in a recent interview he said, “Just because you can give a policy speech, doesn’t mean you can’t also do the other thing.”

The other thing, Mr. Portman said, is firing up Republican crowds, which he did as he campaigned alongside Mr. Romney last month in Ohio. The crowd roared when he bellowed, “Do you agree with me that we can’t afford four more years of Barack Obama?”

Rick A. Lazio, a former member of Congress from New York who roomed with Mr. Portman when they served together in the House, dismissed criticism that Mr. Portman lacked excitement. He said bluntly, “We’re looking for competence.”

In his unsuccessful bid for the Senate in 2000, Mr. Lazio asked his friend to play Mrs. Clinton to help him prepare for his debates, a rare time when Mr. Portman played the role of a woman. He did not wear a wig, Mr. Lazio said, but he perfected her in most every way.

“You just try to make it as unpleasant as possible,” Mr. Portman said with a smile, recalling dozens of debate grilling sessions in which he relentlessly pushed the candidates.

Further testimony to Mr. Portman’s rigor comes from Dick Cheney. He endured months of debate training camps in 2000 and 2004, when Mr. Portman played the parts of Joseph I. Lieberman and John Edwards.

A framed debate poster hanging in Mr. Portman’s office carries a scribbled inscription from Mr. Cheney: “Rob, you were tougher than my opponent.”

Jennifer Steinhauer contributed reporting.

A version of this article appears in print on July 4, 2012, on Page A1 of the New York edition with the headline: Possible No. 2 Knows Ways Of the Capital. Order Reprints|Today's Paper|Subscribe